Pixel driving unit and driving method thereof, and pixel circuit

ABSTRACT

The present disclosure relates to a technical field of display, and more particularly, to a pixel driving unit and a driving method thereof, as well as a pixel circuit comprising the pixel driving unit; the pixel driving unit comprises a driving sub-circuits and a control sub-circuit, wherein the control sub-circuit is connected to a data line, and the driving sub-circuits are connected to the control sub-circuit. In the process of driving the light emitting device, the pixel driving unit can effectively eliminate the nonuniformity due to the threshold voltage of the driving transistor and a phenomenon of image sticking due to the threshold voltage drift, avoid a problem of nonuniform brightness of the active matrix OLED due to the difference of the threshold voltages of the driving transistors thereof between the light emitting devices of different pixel driving units of the active matrix OLED, and improve the driving effect of the pixel driving unit with respect to the light emitting device, and further improve the quality of the active matrix OLED.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of PCT/CN2013/088010 filed onNov. 28, 2013, which claims priority under 35 U.S.C. § 119 of ChineseApplication No. 201310385314.3 filed on Aug. 29, 2013, the disclosure ofwhich is incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to a technical field of display, andparticularly, to a pixel driving unit and a driving method thereof, aswell as a pixel circuit comprising the pixel driving unit.

BACKGROUND

As a light emitting device of current type, organic light-emittingdiodes (OLEDs) have been increasingly applied to active matrix OLED ofhigh performance. Conventional passive matrix OLED requires a shorterdriving time for a single pixel with increase of display size, and thusneeds to increase transient current and increase power consumption. Inthe meanwhile, application of large current may cause a voltage drop onlines of nanometer ITOs (Indium Tin Oxides) to be excessively large andcause the operational voltage of the OLED to be excessively high, andhence the efficiency thereof is decreased. However, active matrix OLED(AMOLED) can address these issues fairly well through inputting OLEDcurrent by switching transistors performing progressive scanning.

In design of the pixel circuit of the AMOLED, the issue that is mainlyfocused on is the nonuniformity of brightness of the OLED devices drivenby the respective AMOLED pixel driving units.

First of all, AMOLED employs thin film transistors (TFTs) to constructthe pixel driving unit so as to provide corresponding driving currentfor the light emitting devices. As known, the Low TemperaturePoly-silicon (LTPS) TFTs or oxide TFTs are mostly used. As compared witha general A-Si TFTs, the LTPS TFTs and the oxide TFTs havecharacteristics of higher mobility and stability, and more suitable forthe application of AMOLED display. However, due to limitation ofcrystallization process, LTPS TFTs fabricated on a glass substrate withlarge area often have a nonuniformity in terms of electrical parameterssuch as threshold voltage, mobility and the like, and this nonuniformitywill be converted into difference in the driving current of the OLEDdevices and difference in brightness and can be perceived by human'seye, that is, a phenomenon of mura. Although the oxide TFTs is prettygood in terms of the uniformity of the process, similar to the A-SiTFTs, in case of being applied a voltage for a long time and hightemperature, a drift will occur in the threshold voltage thereof, andsince the display pictures are different, the drift amounts of thethreshold values of the respective parts of the TFTs of the panel willbe different, which will cause a difference in brightness display; sincethis difference is associated with a previous displayed images, it isgenerally presented as a phenomenon of image sticking.

Since the light emitting device of the OLED is a current driven device,in the pixel driving unit for driving the light emitting device to emitlight, the characteristic of the threshold value of the drivingtransistor thereof has a great impact on the driving current and thefinal displayed brightness. Drift will occur in the threshold value ofthe driving transistor when the driving transistor is subjected to thevoltage stress and illumination, and this drift in threshold value willbe embodied as nonuniformity in brightness in terms of display effect.

In addition, in the pixel circuit of the known AMOLED, in order toeliminate the impact due to the threshold voltage of the drivingtransistor, the structure of the pixel circuit will be generallydesigned to be relatively complex, and this will directly lead todecrease in yield rate of pixel circuit of the AMOLED in themanufacture.

Therefore, in order to address the above issues, the present disclosureprovides a pixel driving unit, a driving method thereof, as well as apixel circuit.

SUMMARY

Embodiments of the present disclosure provide a pixel driving unit and adriving method thereof, as well as a pixel circuit capable of addressingthe issue of the drift in the threshold value of the driving transistorin the known pixel driving unit.

The technical solutions of the present disclosure are realized asfollows: a pixel driving unit comprising driving sub-circuits and acontrol sub-circuit, wherein the control sub-circuit is connected to adata line, and the driving sub-circuits are connected to the controlsub-circuit.

Further, the control sub-circuit comprises a control transistor; thegate and the drain of the control transistor are connected together, andthe source thereof is connected to the data line, and the drain of thecontrol transistor is connected to the driving sub-circuits.

Further, at least three driving sub-circuits are included, wherein eachof the driving sub-circuit includes a scanning signal line, a switchingtransistor, a storage capacitor, a driving transistor and a lightemitting device; the gate of the switching transistor is connected tothe scanning signal line, the source of the switching transistor isconnected to the drain of the control transistor, and the drain of theswitching transistor is connected to the gate of the driving transistorand a first terminal of the storage capacitor respectively; the sourceof the driving transistor is connected to a first voltage terminal and asecond terminal of the storage capacitor respectively, and the drain thedriving transistor is connected to the anode of the light emittingdevice; and the cathode of the light emitting device is connected to asecond voltage terminal.

Further, the light emitting device is an organic light-emitting diode.

Further, each of the control transistor, the switching transistor andthe driving transistor is a field effect transistor of P type.

A driving method for the pixel driving unit as described abovecomprises: applying, by the data line, a data voltage to the source ofthe control transistor, and providing the drain of the controltransistor with the data voltage and the threshold voltage of thecontrol transistor; and applying, by the drain of the controltransistor, the data voltage to the driving sub-circuit together withthe threshold voltage of the control transistor.

Further, the method further comprises the following steps of: turningon, in a storage period, the switching transistor by the scanning signalline; applying, by the drain of the control transistor, the data voltagetogether with the threshold voltage of the control transistor, to thegate of the driving transistor and the storage capacitor through theswitching transistor; and turning off, in a driving period, theswitching transistor by the scanning signal line; keeping the drivingtransistor to be turned on by the storage capacitor, so as to drive thelight emitting device to emit light.

A pixel circuit comprises a plurality of the above data lines eachconnected a plurality of the above pixel driving unit.

A pixel circuit comprises a plurality of the above data lines eachconnected a plurality of the pixel driving unit as described above, andthe above driving method is performed therein.

As compared with the known technique, the embodiments of the presentdisclosure have the following advantages.

Firstly, with a structure in which the gate and drain of the controltransistor are connected with each other, the pixel driving unit of thepresent disclosure allows the drain of the control transistor to applythe data voltage to the driving sub-circuit together with the thresholdvoltage of the control transistor, so as to cancel the threshold voltageof the driving transistor in the driving sub-circuit; in the process ofdriving the light emitting device, it is possible to effectivelyeliminate the nonuniformity due to the threshold voltage of the drivingtransistor and a phenomenon of image sticking due to the thresholdvoltage drift, avoid a problem of nonuniform brightness of the activematrix OLED due to the difference of the threshold voltages of thedriving transistors thereof between the light emitting devices ofdifferent pixel driving units of the active matrix OLED, and improve thedriving effect of the pixel driving unit with respect to the lightemitting device, and further improve the quality of the active matrixOLED.

Secondly, the driving sub-circuit of the present disclosure has acharacteristic of simple structure, and can simplify the overallstructure of the pixel driving unit and the pixel circuit and reduce thedifficulty in manufacturing the pixel circuit; in the meanwhile,combining the driving sub-circuit of simple structure with the controltransistor, it is possible to effectively reduce the difficulty inmanufacturing the pixel driving unit and the pixel circuit andmanufacture cost and improve the yield rate of the pixel circuit, whilethe driving effect for the pixel driving circuit is ensured.

BRIEF DESCRIPTION OF THE DRAWINGS

Hereinafter, the present disclosure will be further explained withreference to the appended drawings and the embodiments.

FIG. 1 is a schematic diagram of circuit connection of a pixel drivingunit according to the present disclosure;

FIG. 2 is a block diagram of steps of a driving method according to thepresent disclosure; and

FIG. 3 is a schematic diagram of circuit connection of a pixel circuitaccording to the present disclosure.

DETAILED DESCRIPTION

Hereinafter, the technical solutions of the embodiments of the presentdisclosure will be clearly and fully described in conjunction with theappended drawings of the embodiments of the present disclosure;obviously, the described embodiments are only a part of the embodimentsof the present disclosure, rather than all of the embodiments. Based onthe embodiments of the present disclosure, all the other embodimentsacquired by those skilled in art without paying any inventive work arewithin the protection scope of the present invention.

With reference to FIG. 1, the present embodiment provides a pixeldriving unit comprising driving sub-circuits and a control sub-circuit,wherein an input terminal of the control sub-circuit is connected to adata line DATA, and an input terminal of the driving sub-circuit isconnected to an output terminal of the control sub-circuit.

The control sub-circuit comprises a control transistor Tc; the gate ofthe control transistor Tc is connected the drain of the controltransistor Tc, the source of the control transistor Tc is connected tothe data line DATA, and the drain of the control transistor Tc isconnected to respective driving sub-circuits.

The pixel driving unit of the present disclosure comprises at leastthree driving sub-circuits; in the following embodiments, a case inwhich there are three driving sub-circuits will be exemplified, and ofcourse, it is also possible to choose more than three drivingsub-circuits as necessary in practice; here, each of the drivingsub-circuits comprises a scanning signal line Scan, a switchingtransistor Ts, a storage capacitor Cs, a driving transistor DTFT and alight emitting device OLED.

Herein, the gate of the switching transistor Ts is connected to thescanning signal line Scan, the source of the switching transistor Ts isconnected to the drain of the control transistor Tc, and the drain ofthe switching transistor Ts is connected to the gate of the drivingtransistor DTFT and a first terminal of the storage capacitor Cs; thesource of the driving transistor DTFT is connected to a first voltageterminal ELVDD and a second terminal of the storage capacitor Cs, andthe drain of the driving transistor DTFT is connected to the anode ofthe light emitting device OLED; the cathode of the light emitting deviceOLED is connected to a second voltage terminal ELVSS.

The first voltage terminal ELVDD of the present disclosure is externallyconnected to an operational power supply, and serves to provide theoperational power supply for the light emitting device OLED. The secondvoltage terminal ELVSS of the present disclosure is connected to thecathode of the light emitting device OLED; the second voltage terminalELVSS serves to provide a reference voltage for the cathode of the lightemitting device OLED. The second voltage terminal ELVSS of the presentdisclosure can be generally selected in a range of −5V to 0V, and can beobtained in accordance with practical adjustment to provide a referencepotential for the above elements, such as zero line, ground line forproviding a zero potential, negative voltage or the like. The lightemitting device OLED of the present disclosure is an organiclight-emitting diode (OLED device).

Each of the driving transistors DTFTs of the present disclosure is afield effect transistor of P type; the field effect transistor of P typeis an enhanced type of field effect (the threshold voltage is positive)or depletion type (the threshold voltage is negative); each of thedriving transistor DTFT, the switching transistor Ts and the controltransistor Tc is the field effect transistor of P type.

With a structure in which the gate and drain of the control transistorTc are connected with each other, the pixel driving unit of theembodiment of the present disclosure allows the drain of the controltransistor Tc to apply the data voltage to the driving sub-circuittogether with the threshold voltage of the control transistor Tc, so asto cancel the threshold voltage of the driving transistor DTFT in thedriving sub-circuit; in the process of driving the light emitting deviceOLED, it is possible to effectively eliminate the nonuniformity due tothe threshold voltage of the driving transistor DTFT and a phenomenon ofimage sticking due to the threshold voltage drift, avoid an issue ofununiform brightness of the active matrix OLED due to the difference ofthe threshold voltages of the driving transistors DTFTs therein betweenthe light emitting devices OLEDs of different pixel driving units of theactive matrix OLED, and improve the driving effect of the pixel drivingunit with respect to the light emitting device OLED, and further improvethe quality of the active matrix OLED.

The driving sub-circuit of this embodiment is a driving sub-circuit of2T1C type, that is, one switching transistor Ts, one driving transistorDTFT and one storage capacitor Cs are included; it has a characteristicof simple structure, and can simplify the overall structure of the pixeldriving unit and the pixel circuit and reduce the difficulty inmanufacturing the pixel circuit; in the meanwhile, combining the drivingsub-circuit of simple structure with the control transistor, it ispossible to effectively reduce the difficulty in manufacturing the pixeldriving unit and the pixel circuit and manufacture cost and improve theyield rate of the pixel circuit, while the driving effect for the pixeldriving circuit is ensured.

With reference to FIG. 2, the present disclosure further provides adriving method of the pixel driving unit described above, whichcomprises a voltage applying period of applying, by the data line DATA,the data voltage V_(data) to the source of the control transistor Tc,allowing the drain of the control transistor Tc to have the data voltageV_(data) and the threshold voltage V_(th-control) of the controltransistor Tc, and applying, by the drain of the control transistor Tc,the data voltage V_(data) to the driving sub-circuit together with thethreshold voltage V_(th-control) of the control transistor Tc.

The driving method further comprises a storage period for the drivingsub-circuit, during which the scanning signal line Scan is at the lowpotential, and the switching transistor Ts is turned on; the drain ofthe control transistor Tc applies the data voltage V_(data) to the gateof the driving transistor DTFT and the first terminal of the storagecapacitor Cs together with the threshold voltage V_(th-control) of thecontrol transistor Tc via the switching transistor Ts, and the datavoltage V_(data) and the threshold voltage V_(th-control) of the controltransistor Tc are stored in the first terminal of the storage capacitorCs.

The driving method further comprises a driving period for the drivingsub-circuit, during which the scanning signal line Scan is at the highpotential, and the switching transistor Ts is turned off; the firstterminal of the storage capacitor Cs is at the low potential, and thedata voltage V_(data) and the threshold voltage V_(th-control) of thecontrol transistor Tc are maintained to the gate of the drivingtransistor DTFT; at this timing, the voltage of the gate of the drivingtransistor DTFT is equal to V_(data)+V_(th-control), so as to keep thedriving transistor DTFT to be turned on; the first voltage terminalELVDD applies the operational voltage VDD to the anode of the lightemitting device OLED through the driving transistor DTFT so as to drivethe light emitting device OLED to emit light.

At this timing, the voltage of the gate of the driving transistor DTFTis maintained at V_(data)+V_(th-control), and the voltage of the sourceof the driving transistor DTFT is an operational voltage of VDD;therefore, the driving voltage Vgs outputted from the drain of thedriving transistor DTFT is calculated by a formula ofVgs=V_(data)+V_(th-control)−VDD; the driving current outputted to thelight emitting device OLED via the driving transistor DTFT is calculatedby a formula ofI _(OLED)=½·K·[V _(gs) −V _(th-drive)]²where, K is a current constant related to the driving transistor OLED,and V_(th-drive) is the threshold voltage of the driving transistorOLED; it can be known, from substituting the Vgs into the formula of thedriving current I_(OLED), that the driving current I_(OLED) outputted tothe light emitting device OLED via the driving transistor DTFT isexpressed by a formula ofI _(OLED)=½K·[V _(data) +V _(th-control) −V _(DD) −V _(th-drive)]²

Since an issue of short range uniformity between the threshold voltagesof the control transistor Tc and the respective switching transistors Tsas well as the respective driving transistors DTFT is considered in thedesign of the pixel driving unit, in each of the pixel driving units,the threshold voltages of the control transistor Tc, switchingtransistor Ts and the driving transistor DTFT manufactured with the samedesign rule are approximately equal to each other; therefore, thethreshold voltage of the control transistor Tc and the threshold voltageof the driving transistor DTFT described in the above formulas arecancelled with each other to obtain the following formula ofI _(OLED)=½K·[V _(data) −V _(DD)]²

It can be known from the above calculations that the driving currentI_(OLED) flowing through the driving transistor DTFT is only related tothe V_(data) and the VDD, and is independent of the threshold voltageV_(th-drive) of the driving transistor DTFT. Therefore, even if theV_(th-drive) is lower than zero, it can be well compensated, and theimpact of the nonuniformity and the drift of the threshold voltage ofthe driving transistor DTFT is basically eliminated. With the pixelcircuit of the embodiments of the present disclosure, no matter withrespect to the driving transistor of the enhanced type or depletiontype, the impact of the nonuniformity of the threshold voltage can beeliminated, and thus the nonuniformity of brightness of the lightemitting device can be greatly compensated and the application thereofis much wider.

With reference to FIG. 3, the present disclosure also provides a pixelcircuit comprising a plurality of the above data lines DATAs, and eachof the data lines is connected a plurality of the pixel driving unitsPUs as described above; Here, the source of the plurality of the controltransistors Tc on the same column are connected to a same data lineDATA, and the driving sub-circuits on the same row are connected to asame scanning signal line Scan.

It should be explained, the source and the drain of all transistors inthe embodiments of the present disclosure are not distinguished fromeach other; for example, the source of the driving transistor can bealso referred to as the drain of the driving transistor, andcorrespondingly, in this case, the drain of the driving transistor isreferred to as the source of the driving transistor; that is to say, forthe two terminals other than the gate, one is the source, and the otheris the drain.

What is claimed is:
 1. A pixel driving unit comprising a plurality ofdriving sub-circuits and a control sub-circuit, wherein the controlsub-circuit is connected to a data line, and the plurality of drivingsub-circuits are connected to the control sub-circuit; wherein thecontrol sub-circuit comprises a control transistor; a gate and a drainof the control transistor are connected together, a source thereof isconnected to the data line, and a drain of the control transistor isconnected to the plurality of driving sub-circuits; wherein each of theplurality of driving sub-circuits includes a scanning signal line, aswitching transistor, a storage capacitor and a driving transistor; agate of the switching transistor is connected to the scanning signalline, a source of the switching transistor is connected to the drain ofthe control transistor directly, and a drain of the switching transistoris directly connected to the gate of the driving transistor and a firstterminal of the storage capacitor; and a source of the drivingtransistor is connected to a second terminal of the storage capacitorand a first voltage terminal, wherein the plurality of drivingsub-circuits are connected to different scan signal lines respectively;wherein a sum of a data voltage at the data line and a threshold voltageof the control transistor is applied to the gate of the drivingtransistor when the switching transistor is turned on so as to cancel aneffect of a threshold voltage of the driving transistor; wherein thepixel driving unit is configured to drive a single pixel comprising aplurality of sub-pixels, and the driving transistor included in each ofthe plurality of driving sub-circuits is configured to drive each of theplurality of sub-pixels; the control transistor is shared by theplurality of driving sub-circuits and is configured to have a samethreshold voltage as that of the driving transistor in each of theplurality of driving sub-circuits.
 2. The pixel driving unit accordingto claim 1, wherein each of the plurality of driving sub-circuitsfurther comprises a light emitting device; an anode of the lightemitting device is connected to the drain of the driving transistor; anda cathode of the light emitting device is connected to a second voltageterminal.
 3. The pixel driving unit according to claim 2, wherein thelight emitting device is an organic light-emitting diode.
 4. The pixeldriving unit according to claim 3, wherein each of the controltransistor, the switching transistor and the driving transistor is afield effect transistor of P type.
 5. A driving method for a pixeldriving unit comprising a plurality of driving sub-circuits and acontrol sub-circuit, wherein the control sub-circuit is connected to adata line, and the plurality of driving sub-circuits are connected tothe control sub-circuit; wherein the control sub-circuit comprises acontrol transistor; a gate and a drain of the control transistor areconnected together, a source thereof is connected to the data line, anda drain of the control transistor is connected to the plurality ofdriving sub-circuits; wherein each of the plurality of drivingsub-circuits includes a scanning signal line, a switching transistor, astorage capacitor and a driving transistor; a gate of the switchingtransistor is connected to the scanning signal line, a source of theswitching transistor is connected to the drain of the control transistordirectly, and a drain of the switching transistor is directly connectedto the gate of the driving transistor and a first terminal of thestorage capacitor; a source of the driving transistor is connected to asecond terminal of the storage capacitor and a first voltage terminal;wherein each of the plurality of driving sub-circuits further comprisesa light emitting device; an anode of the light emitting device isconnected to the drain of the driving transistor; and a cathode of thelight emitting device is connected to a second voltage terminal; whereinthe plurality of driving sub-circuits are connected to different scansignal lines respectively; the method comprises: applying, by the dataline, a data voltage to the source of the control transistor, andproviding the drain of the control transistor with the data voltage anda threshold voltage of the control transistor; and applying, by thedrain of the control transistor, the data voltage together with thethreshold voltage of the control transistor to the plurality of drivingsub-circuit; wherein a sum of a data voltage at the data line and athreshold voltage of the control transistor is applied to the gate ofthe driving transistor when the switching transistor is turned on so asto cancel an effect of a threshold voltage of the driving transistor;wherein the pixel driving unit is configured to drive a single pixelcomprising a plurality of sub-pixels, and the driving transistorincluded in each of the plurality of driving sub-circuits is configuredto drive each of the plurality of sub-pixels; the control transistor isshared by the plurality of driving sub-circuits and is configured tohave a same threshold voltage as that of the driving transistor in eachof the plurality of driving sub-circuits.
 6. The driving methodaccording to claim 5, further comprising: turning on, in a storageperiod, the switching transistor by the scanning signal line; applyingby the drain of the control transistor, together with the thresholdvoltage of the control transistor, the data voltage to the gate of thedriving transistor and the storage capacitor via the switchingtransistor; and turning off, in a driving period, the switchingtransistor by the scanning signal line; keeping the driving transistorto be turned on by the storage capacitor, so as to drive the lightemitting device to emit light.
 7. A pixel circuit comprising a pluralityof data lines, wherein each of the data lines is connected to aplurality of the pixel driving units each comprising a plurality ofdriving sub-circuits and a control sub-circuit, wherein the controlsub-circuit is connected to a data line, and the plurality of drivingsub-circuits are connected to the control sub-circuit; wherein thecontrol sub-circuit comprises a control transistor; a gate and a drainof the control transistor are connected together, a source thereof isconnected to the data line, and a drain of the control transistor isconnected to the plurality of driving sub-circuits; wherein each of theplurality of driving sub-circuits includes a scanning signal line, aswitching transistor, a storage capacitor and a driving transistor; agate of the switching transistor is connected to the scanning signalline, a source of the switching transistor is connected to the drain ofthe control transistor directly, and a drain of the switching transistoris directly connected to the gate of the driving transistor and a firstterminal of the storage capacitor; a source of the driving transistor isconnected to a second terminal of the storage capacitor and a firstvoltage terminal; wherein the plurality of driving sub-circuits areconnected to different scan signal lines respectively; wherein a sum ofa data voltage at the data line and a threshold voltage of the controltransistor is applied to the gate of the driving transistor when theswitching transistor is turned on so as to cancel an effect of athreshold voltage of the driving transistor; wherein each of theplurality of the pixel driving units is configured to drive a singlepixel comprising a plurality of sub-pixels, and the driving transistorincluded in each of the plurality of driving sub-circuits is configuredto drive each of the plurality of sub-pixels; the control transistor isshared by the plurality of driving sub-circuits and is configured tohave a same threshold voltage as that of the driving transistor in eachof the plurality of driving sub-circuits.
 8. The pixel circuit accordingto claim 7, wherein each of the plurality of driving sub-circuitsfurther comprises a light emitting device; an anode of the lightemitting device is connected to the drain of the driving transistor; anda cathode of the light emitting device is connected to a second voltageterminal.
 9. The pixel circuit according to claim 8, wherein the lightemitting device is an organic light-emitting diode.
 10. The pixelcircuit according to claim 9, wherein each of the control transistor,the switching transistor and the driving transistor is a field effecttransistor of P type.